1. Technical Field
The present invention relates to a member having an antireflection structure. More particularly, the present invention relates to a member having an antireflection structure capable of improving the image characteristics of optical apparatuses, for example.
2. Description of the Background Art
Most of optical elements and optical components being used for various uses are requested to have an antireflection function to prevent the reflection of light. FIG. 5 is a schematic cross-sectional view showing the configuration of an imaging optical device being used generally and also showing the cross-sectional structure of an imaging optical device 50 along a plane including an optical axis 51. In FIG. 5, an aperture diaphragm 53, a lens element 54, a lens element 55 and a lens element 56 on which a lens-holding member 57 is mounted, these being disposed coaxially along the optical axis 51, are provided inside a lens barrel 52 constituting the imaging optical device 50.
In the imaging optical device 50 configured as described above, if unnecessary light is reflected, the reflected light of this unnecessary light may become stray light occasionally. The stray light causes ghost and flare, thereby degrading image quality. Hence, unnecessary light, such as luminous flux entering from the lens element 54 and having an angle being equal to or more than the comprehensive angle of view thereof and stray light caused by the reflection at the surfaces of the lens elements 54, 55 and 56, is guided to an aperture portion 53a formed in the aperture diaphragm 53 or to an aperture portion 57a formed in the lens-holding member 57, whereby the reflection of the unnecessary light is prevented. Furthermore, in recent years, for the purpose of further preventing the reflection of the unnecessary light, antireflection treatment is performed to form an antireflection film, such as a single-layer film comprising a layer having a low refractive index or a multi-layer film comprising layers having a low refractive index and layers having a high refractive index, these layers being laminated, on optically functioning faces of optical components, such as the lens elements 54, 55 and 56, the lens barrel 52, and the aperture diaphragm 53, by carrying out evaporation, sputtering, coating or the like (for example, refer to Japanese Laid-Open Patent Publication No. 2001-127852).
This kind of antireflection film has been widely used because it can be formed using a general method, such as evaporation or sputtering. However, such a method requires complicated processes to accurately control the optical film thickness of the antireflection film, thereby being desired to be improved in productivity and cost. In addition, because the antireflection film has wavelength dependence, its antireflection effect at non-predetermined wavelengths becomes small. In particular, it is very difficult to attain excellent antireflection at the entire range of visible light, required in imaging optical devices. Furthermore, the antireflection film also has incident angle dependence in which the antireflection effect becomes smaller as the incident angle becomes larger. For these reasons, an antireflection treatment method capable of improving the wavelength dependence and the incident angle dependence is desired to be developed.
As a method of solving the problem regarding the wavelength dependence and the incident angle dependence, there is attracting attention for a technology of forming a structure in which structural elements having a minute concavoconvex shape are arranged at a submicron period, for example, referred to as an antireflection structure, on the optically functioning face of an optical element or an optical component, in recent years.
When this kind of antireflection structure is formed on the optically functioning face of the optical element or the optical component, the distribution of the refractive index on the optically functioning face becomes to change smoothly. Hence, almost all of incident light having wavelengths being equal to or more than the period at which the structural elements having a concavoconvex shape are arranged enter the inside of the optical element or the optical component. Therefore, the reflection of light on the optically functioning face can be prevented. Furthermore, in the case that the antireflection structure is formed on the optically functioning face, the antireflection effect does not become much smaller even if the incident angle of the incident light becomes larger. Therefore, it is possible to solve the problem regarding the wavelength dependence and the incident angle dependence of the antireflection film by forming the antireflection structure on the optically functioning face of the optical element or the optical component.
In the imaging optical device 50 configured as described above, although the antireflection structure is formed on the optically functioning faces of various kinds of members thereof, the antireflection structure is not formed on an inner wall of the aperture portion 53a in the aperture diaphragm 53 and an inner wall of the aperture portion 57a in the lens-holding member 57. This is based on the following reason: because plate-like portions of the aperture diaphragm 53 and the lens-holding member 57, in which the aperture portion 53a and the aperture portion 57a are formed, respectively, are very thin, approximately 1 mm or less, the reflection of light on the inner walls of the aperture portions 53a and 57a is ignored in the optical design of the device.
No serious problems have been caused in conventional imaging optical devices, even though they are configured on the basis of the above optical design. However, in recent years, as imaging optical devices become more compact, the refractive index of the incident light at the inner walls of the aperture portions in the aperture diaphragm and the lens-holding member changes abnormal, and the distribution of the refractive index at the optically functioning face does not change smoothly. As a result, unnecessary light is reflected slightly, and ghost and flare occur, thereby causing a problem of degrading image characteristics.